It is believed that the pertinent state-of-the-art is represented by the following patent literature: U.S. Pat. No. 7,995,730, U.S. Pat. No. 7,436,943 and U.S. Pat. No. 7,602,894; US patent application Ser. No. US2010/054444 and US2012/328089; GB patent application Ser. No. GB2454886; European patent application Ser. No. EP2073521 and EP2448227 as well as by international patent applications having Publication No. WO2007/053768 and WO2010/135000.
It is believed that the pertinent state-of-the-art is represented by the following non-patent literature: Requests for Comments 3261 and 5627; Technical Specification of 3rd Generation Partnership Project—3GPP TS 23.228 V12.0.0 (2013-03); Session Initiation Protocol (SIP) and Session Initiation Protocol for Instant Messaging (SIMPLE) standards collection.